Pavement markings--e.g., paints, tapes, and individually mounted articles--are used commonly to guide and direct motorists traveling along a roadway. During the daytime, the markings are usually sufficiently visible to guide motorists. At night, however, when the primary source of illumination is the vehicle headlights, the marking may not be sufficiently bright to guide the motorist unless it retroreflects light. Retroreflective pavement markings have the ability to return substantial quantities of incident light in the direction from which the light originated. For this reason, retroreflective pavement markings are commonplace on roadways.
Many retroreflective pavement markings, such as lines on highways, are made by dropping optical elements, such as glass beads, onto the line while it is still tacky. Others are made by securing optical elements to a base sheet that contains pigments and fillers. Securement of the optical elements can be achieved either by embedding the elements into the base sheet or by securing the elements to the base sheet with a binder. The pigments and fillers typically are dispersed throughout the base sheet for several reasons, such as reducing cost, improving durability, and providing conformability. Pigments also enhance pavement marking visibility and can play a role in the retroreflective mechanism.
Incident light retroreflects from pavement markers in the following manner. First, the incident light passes through the optical elements to strike the pigments in the base sheet or in the bonding material of the marker. The pigments then scatter the incident light back into the microspheres, and the microspheres redirect a portion of the scattered light back towards the light source. For effective retroreflection from pavement markings, especially under wet conditions, the microspheres preferably are elevated above the pavement surface so that they will not be submerged in water during a rainy period.
An example of a pavement marker where the microspheres are elevated is disclosed in U.S. Pat. No. 4,988,555 (Hedblom). This pavement marker contains a pattern of protrusions that have vertical surfaces where microspheres are embedded so as to be elevated above the pavement surface. The microspheres are elevated and are oriented vertical to the incident light, providing for more efficient retroreflection. Because of their elevated position, the microspheres often are not completely submerged in water. The protrusions also allow the retroreflective portions of the marking to drain more efficiently and recover retroreflective performance more quickly after cessation of rainfall.
While patterned pavement markers have become very useful articles, their manufacturing process is somewhat complex. For example, as disclosed in U.S. Pat. No. 4,988,541 (Hedblom), the integral protrusions are created by embossing a sheet of polymeric material using an embossing roll that has a predetermined pattern of recesses. As the polymeric material fills the recesses in the embossing roll, protrusions having set pattern, dimensions, and spacing are formed. After the embossing process, binder materials are carefully placed on the protrusions in a manner that keeps the binder from flowing into the valleys between the protrusions. Microspheres and/or skid resistant particles are then secured to the binder material. Not only is this process somewhat complicated, but changing the protrusion pattern or shape, size, or spacing requires changing the embossing roll, which typically requires labor and extended amounts of time. With each different protrusion pattern, there must be a corresponding embossing roll.
Japanese Patent Kokoku (B2) No. HEI 5[1993]-33661 (Shinmi et al.) discloses a sheet for road signs having optional convex molded shapes on the sheet's surface. Anchored on the convex molded shapes are reflective materials. The sheet comprises thermoplastic polymers and additives, such as fillers, pigments, plasticizers, and reflective materials. The convex molded shapes are made by forcing molten sheet material into a molding roller. The convex shapes are an integral part of the sheet. The molding roller determines features such as size, shape, and spacing of the convex shapes so that changes to those features cannot be made readily without changing the configuration of the molding roller, a situation similar to U.S. Pat. No. 4,988,541 (Hedblom).
Assignee's pending U.S. patent application Ser. No. 08/562,041 (filed Nov. 22, 1995) discloses a pavement marker comprising a base sheet, a discontinuous polymeric layer adhered on the base sheet, and a plurality of particles, such as microspheres and skid resistant particles, partially embedded in the polymeric layer. The polymeric layer is a thermoset polymer comprising a blocked isocyanate crosslinker and is applied to the base sheet as a pattern, e.g., a repeating pattern of hexagons, by a continuous process such as screen printing. Although the pavement marker is very useful and although the manufacturing process is generally streamlined, the pattern of the polymeric layer is predetermined by the screen printing method and cannot be changed readily without equipment changes.
An alternate method to elevate the optical elements above the pavement surface is to use retroreflective elements or aggregates having a core material that is coated with a multitudes of microspheres. Examples of such elements are disclosed in EP Patent No. 565,756 A2; U.S. Pat. Nos. 3,043,196; 3,171,827; 3,175,935; 3,274,888; 3,418,896, 3,556,637; 4,983,458; and Assignee's pending U.S. patent application Ser. No. 08/503,532, filed Jul. 18, 1995 now U.S. Pat. No 5,750,191. Although these retroreflective elements are extremely useful, some are not easily manufactured.
A need still exists for other retroreflective articles that provide good recovery of retroreflectivity after exposure to wet conditions but can be manufactured through a streamlined process for making such articles.